This invention relates in general to diamond crystal growing techniques and, more particularly, to methods for growing diamond thin films on selected substrates. Diamond films exhibit numerous properties which are desirable in the optics and electronics industries. For example, diamond thin films are hard, transparent, chemically inert, electrically insulative or semi-conductive and exhibit relatively low friction.
In the recent past, several approaches have been employed to grow diamond thin films on various substrates. For practical applications in optics and electronics, the synthesis of large area, relatively smooth diamond thin films on silicon or other suitable substrates is very desirable. Unfortunately, due to lattice mismatch and random nucleation, generally only relatively rough poly-crystalline diamond films have been deposited on silicon substrates in the past. Such polycrystalline diamond films are typically formed by inter-growth of disjoint diamond crystallites, thus resulting in a very rough surface such as that depicted in FIG. 1 wherein diamond crystallites 2 are shown on a substrate 4. The morphology of these diamonds films is usually dominated by (111) or (100) oriented faces, depending on the particular deposition conditions selected. These (111) and (100) faces are generally randomly oriented in direction.
Several low-pressure chemical vapor deposition (CVD) techniques have been employed to achieve deposition of diamond films. For example, thermal chemical vapor deposition, plasma chemical vapor deposition, electron-assisted chemical vapor deposition have all been used for this purpose. Generally, CVD diamond deposition techniques involve introducing a variety of hydrocarbon gasses into a contained environment and subjecting a target substrate to a plasma in the presence of such gasses within the contained environment. Methane/hydrogen mixtures are commonly used as the aforementioned hydrocarbon gasses.
Other techniques such as RF sputtering and ion beam techniques have also been employed to achieve diamond deposition. Moreover, it is also known to irradiate a hydrocarbon vapor with a high powered pulsed laser to achieve deposition of diamond film at the point where the laser contacts a substrate.
Badzian has reported that, in a filament-assisted CVD system at temperature of 900 degrees C. or lower, (111) faces dominated the morphology of the deposited diamond film and, at temperatures of 1000 degrees C. or greater, (100) faces dominated the morphology. X Ray Analysis 31, 113 (1988) and Material Science Bulletin 23, 531, (1988).
It has also been reported by Kobashi et al. that, with a microwave CVD system in which the substrate temperature is 800 degrees C. and the methane concentration is 0.4% or less, (111) faces dominated the morphology of the deposited diamond film. In contrast, when the methane concentration was between 0.4% and 1,2%, (100) faces became the dominant faces of the diamond film morphology. Journal of Vacuum Science, A6, 1816 (1988) and Phys. Rev. B38, 4067 (1988).
When combining hot filament and microwave plasma CVD techniques, Wild et al. observed a preferential alignment of (110) planes perpendicular to the growth direction of the diamond film in their CVD system. Journal of Applied Physics 68, 973 (1990).
In contrast, Haubner and Lux found that, in their microwave CVD system, (100) faces dominated the morphology of the resultant diamond film when a 0.3% methane concentration was employed at 600 degrees C. At temperatures in excess of 750 degrees C., they found that (111) faces started to dominate the morphology of the diamond film. Int. J. Refract. Hard Mat. 6, (1987).
Also, Jeng et al. report the oriented cubic nucleation of diamond and local epitaxial diamond growth on a (100) silicon substrate using microwave CVD technique. Applied Physics Letters 56, 1968 (1990).
In yet another deposition technique, Ravi and Koch employed an acetylene torch to deposit diamond and diamond-like thin film on a molybdenum substrate. Ravi and Koch report that both the dangling bonds and the hydrogen content in the diamond thin film increase the nucleation rate. They also report their belief that this increase in nucleation rate leads to multiple twins and dendritic growth. Applied Physics Letters 57, 348 (1990).
Chang et al. report diamond crystal growth by plasma chemical vapor deposition employing a microwave discharge tube reactor to generate a plasma. CH.sub.4, H.sub.2 and O.sub.2 gasses were fed into the discharge tube. When a silicon wafer was situated within the plasma region in the reactor, diamond crystals were grown in the plasma region of the wafer. J. Applied Physics, 63, 1744 (1988).
Unfortunately, the above referenced diamond deposition methods are not known to form large area, relatively smooth diamond thin film layers. As remarked earlier, such large area relatively smooth diamond layers are very desirable on substrates such as silicon, for example, for various optics and electronics applications.